The Land Transport Authority (LTA), together with our main contractor Alstom, have reviewed the findings of the forensic investigations for the damaged 22kV cables and faulty trip coil which had led to the disruption on the North-South and East-West Lines (NSEWL) on 14 October 2020.

2        Following the incident, the sample of the power cables and trip coils had been sent to both the original equipment manufacturer and independent laboratories for detailed examination, to determine the root causes for their failure.

Damaged cables

3        Power cables comprise multiple layers of different materials. Forensic analysis indicated that metallic screen layer in the damaged 22kV power cables was defective and had cut into the cables’ internal insulation layer, resulting in a short circuit. This is likely due to an inherent manufacturing defect or mishandling during installation.

4        As a precautionary measure, Alstom had previously committed to replacing the affected 22kV power cables with higher specification cables. Cable replacement works have commenced, with Full Sunday Closures taking place at selected stations along the East-West Line from 14 March until 23 May 2021 to speed up the works.

Faulty trip coil

5        Investigations revealed that the trip coil failure was caused by rust developing on the surface of the plunger rod, which is a critical component of the trip coil. The rust is likely formed due to an inadequate protective coating of the plunger rod for a particular manufacturing batch. This prevented the movement of the plunger rod. During the 14 October 2020 incident, when the affected section of damaged cables between Tuas Link and Tuas West Road stations resulted in a fault, the trip coil malfunctioned. As such, it was unable to isolate the fault, resulting in the train service disruption. Please refer to the Annex for a diagram depicting the plunger rod mechanism in a trip coil.

6        Taking into account the investigation findings, LTA has worked with Alstom to design and manufacture new trip coils which are more resistant to rust. Once the newly designed trip coils are manufactured, all 113 trip coils which Alstom had already changed-out in December 2020 will be replaced. To further mitigate the risk of a future recurrence, the humidity setting within the circuit breakers will also be lowered, so as to reduce the ambient humidity and minimise the risk of rusting. In the interim, SMRT has stepped up the frequency of their maintenance checks for the trip coils.